Single crystal selenium rectifier



Get. 14, 1969 c GR|FF|TH$ ET AL 3,473,095

SINGLE CRYSTAL SELENIUM RECTIFIER Filed A g. 22, 1966 3 Sheets-Sheet 1 DIFFUSION PUMP SUBSTRATE SUBSTRATE Y HEATER & I

THERMOCOUPLE i, (I: SHUTTER 3 APERTURE s un'cs SOURCE FIG.I

' 'spnms [DADED CARBON ooNTACr l6 5 LavuR r Tc SUBSTRATE c'ms SELENIUM LA 6070) PLANE FIGZ Oct. 14, 1969 c. H. GRIFFITHS ET AL 3,473,095

SINGLE CRYSTAL SELENIUM RECTIFIER Filed Aug. 22, 1966 3 Sheets-Sheet 2 VERTICAL SENSITIVITY= 0.2mA/cm HORIZONTAL SENSITIVITY: 0.5 V/cm (A) AFTER FORMING FoR l5 MINUTES AT 4 v PEAK-PEAK.

FREQUENCYI 2OC.F?S.

VERTICAL SENSITIVITY: I I0 mA/cm HORIZONTAL sENsITIvITY= 20 V/cm (B)AFTER TOTAL FORMING TIME OF 24 HOURS.

FINAL FORMING CARRIED OUT FoR 3 HouRs AT 90 VOLTS PEAK REcTIFIED FIG. 3

c. H. GRIFFITHS ET AL 3,473,095

SINGLE CRYSTAL SELENIUM RECTIFIER Oct. 14, 1969 5 Sheets-Sheet 5 Filed Aug. 22. 1966 VERTICAL SENSITIVITY 5 mA/cm.

I II IIJI G N I W W 4 A I .7 m E R O F E B I HORIZONTAL SENSITIVITY O5 V/cm.

FREOUENCYI 60 CBS.

L A E T H V SENSITIVITY= 5O mA/cm.

HORIZONTAL SENS|TIV|TY= (B) AFTER TOTAL FORMING TIME OF 9| HOURS.

FINAL FORMING CARRIED OUT AT 50 V PEAK RECTIFIED A.C. FOR 48 HOURS.

FIG. 4

United States Patent 3,473,095 Patented Oct. 14, 1969 U.S. Cl. 317-241 12 Claims ABSTRACT OF THE DISCLOSURE A selenium rectifier utilises selenium in the form of a single-crystal film.

This invention relates to selenium rectifiers, and according to the present invention there is provided a selenium rectifier comprising a single crystal film of selenium.

The rectifier preferably comprises a single crystal film of selenium deposited epitaxially on one electrode of the rectifier, and this electrode is preferably a single crystal of tellurium. The crystalline orientation of the selenium single crystal film is the same as that of the tellurium plane on which it is deposited.

A counter electrode of cadmium or cadmium alloy for example is deposited as a film on the selenium preferably from the vapour. The single crystal of selenium is preferably prepared in the manner described in co-pending application Ser. No. 541,551.

An example of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a diagram of apparatus for use in a vacuum chamber for making a selenium rectifier,

FIG. 2 is a diagram illustrating the layer structure of a selenium rectifier made in the apparatus of FIGURE 1,

FIGS. 3(a) and 3(b) show the current/voltage characteristics of an undoped single crystal selenium rectifier after two periods of forming,

FIGS. 4(a) and 4(b) show current/voltage characteristics of a doped single crystal selenium rectifier before and after forming Referring now to FIGURE 1, a tellurium single crystal, prepared by one of a number of techniques which are known in the art, is cleaved to expose a (1010) face (or the so-called prism face). This face is then ground flat and polished using conventional techniques. Grinding first with emery paper and polishing with a tin oxide suspension in water has been found to be satisfactory. The disturbed surface layer of the tellurium resulting from the polishing is then removed by annealing the crystals at a temperature of 185 C. for two hours under vacuum. These annealing conditions are not critical. The cleaved surface may also be directly annealed without polishing. A film of selenium is next deposited on the tellurium surface in the manner described in the aforementioned copending application. If the deposition is done by evaporation, the thickness of the film can be controlled by the rate and time of evaporation. A rate of 0.2 micron thickness per minute has been found to be satisfactory. The thickness of the film for rectifier purposes may be of the order of one micron, but thicker films should also prove satisfactory. The usual tests will determine the preferred range of thicknesses. It may be noted that in the epitaxial method of selenium film growth subsequent heat treatment, normally required to complete the crystallization, is unnecessary.

Cadmium or some other suitable counter electrode alloy is then deposited on the selenium by any of the conventional methods. A method found to be satisfactory is to mask oif the edges of the selenium films with collodion and to evaporate cadmium onto the selenium under vacuum. Alternatively the cadmium may be deposited immediately following the deposition of the selenium without breaking the vacuum, using the apparatus shown in FIG. 1. In this apparatus the thermocouple 1, mask 2, and shutter 3 can be rotated and moved vertically from outside the evaporation chamber. The two apertures 6 and 4 over the cadium and selenium sources are arranged such that the shutter 3 can cover either aperture or both. This allows the substrate 5 to be annealed without contaminating the evaporants, selenium to be evaporated without contaminating the cadmium, and the selenium vapour beam to be cut off and cadmium substituted. A passage 12 leads from the region of the sources to a diffusion pump.

In the preparation of a rectifier with the apparatus of FIGURE 1, the telleurium substrate is annealed by a substrate heater 11 with the thermocouple 1 in place and the mask 2 in an inoperative position. The shutter 3 is moved to expose the selenium source aperture 4 and the selenium layer deposited. The thermocouple 1 is removed from the substrate 5 and mask 2 moved into position over the selenium layer to expose a central portion of the layer. The shutter 3 is moved to expose the cadmium aperture 6 and to cover up the selenium aperture 4 so that the cadium is deposited through the mask 2 onto the selenium layer. The mask 2 prevents the cadmium from depositing on the edges of the selenium layer and contacting the tellurium directly. This would cause short circuits in the rectifier.

The product obtained by this method is, as illustrated in FIGURE 2, a (1010) selenium single crystal film 14 bonded underneath to a (1010) single crystal of telluriurn 5 and bonded on top to a cadmium layer 16. The c axis of the crystal lies parallel to the selenium/tellurium interface. Suitable electrodes may then be connected to the device. In laboratory tests it was shown that copper wire may be soldered directly to tellurium using, for example, a tinbismuth-antimony solder. Contact to the counter electrode may be made by means of a spring loaded graphite block 17, as shown in FIGURE 2. However, many other methods of attaching leads are available and in fact the conventional commercial means used in rectifier construction may be employed.

Electrical forming is carried out in a similar way to that employed in conventional selenium rectifiers. A small reverse rectified alternating voltage is applied to the unit and the voltage is then progressively increased with time. The process is stopped when the current/voltage characteristics do not change further with time. Such a characteristic can be displayed visually on the screen of an oscilloscope. The reverse breakdown voltage which is the voltage at which the reverse current starts to increase rapidly, increases considerably during the forming process.

Typical conditions used in the fabrication of two single crystal rectifiers are summarized in Table I. In unit 12 the selenium was undoped, While in unit 17 the material was doped with 200 parts per million by weight of chlor1ne.

FIGURE 3(a) shows the dynamic current/voltage characteristic for an undoped single crystal selenium rectifier unit after forming for 15 minutes with 4 volts peakto-peak alternating current at 20 cycles per second.

FIG. 3(b) shows the characteristics of the same rectifier after a total forming time of 24 hours, in which the final forming was carried out for 3 hours at volts peak rectified alternating current.

FIGS. 4(a) and 4(b) show similar characteristics for a single crystal selenium rectifier unit which has been doped with chlorine.

FIG. 4(a) shows the characteristics before forming; and

FIG. 4(1)) shows the characteristics of the same unit after total forming time of 91 hours, in which the final forming was carried out at 50 volts peak rectified alternating current at 60 cycles per second for 48 hours. The eifect of the chlorine doping is to reduce the dynamic reverse breakdown voltage from 70 to 40 volts and to decrease the specific area dynamic forward resistance from about 700 to 20 ohm cm.

It is expected that the single crystal selenium film may be oriented on the (0001) plane which is the basal plane, and is in this case preferably deposited directly on a (0001) plane of a single crystal tellerium as shown in FIGURE 5. This method has been described in the aforementioned co-pending patent application.

The counter electrode of the rectifiers may also be applied by deposition of an alloy using such methods as spraying or dipping or by the direct application of a single crystal or polycrystalline layer of cadmium selenide.

Suitable dopants for the selenium or for the counter electrode to obtain controlled variations in the forward and reverse electrical characteristics include halogens and thalium.

orientation as the said surface of the electrode on which it is grown.

4. A selenium rectifier as claimed in claim 3, and in which the first electrode is of tellurium.

5. A selenium rectifier as claimed in claim 4, and in which the selenium is deposited on the said face which is oriented in the (lOTO) plane.

6. A selenium rectifier as claimed in claim 4, and in which the single-crystal tellurium face of the first electrode is prepared by epitaxial growth on a single-crystal material.

7. A selenium rectifier as claimed in claim 6, and in which the single-crystal tellurium face is formed by deposition from the vapour phase.

8. A selenium rectifier as claimed in claim 1, and in which the second electrode is deposited from the vapour phase as a film onto the selenium.

9. A selenium rectifier as claimed in claim 1, and in which the second electrode is deposited onto the selenium by spraying.

10. A selenium rectifier as claimed in claim 1, and in which the selenium film is doped to provide suitable differences between the forward and reverse electrical characteristics of the rectifier.

TABLE I.DETAILS OF PREPARATION OF SELENIUM SINGLE CRYSTAL RECTIFIER UNITS Selenium deposition conditions Approx. Forming thickness Unit Temp. Boat Dist. to of Se film Rev. Time No. Selenium used Base Material 0.) charge (g.) S(ubst). (micron) voltage (hr.) Remarks 12- OCR HP Se Polished (ltfiO) 94 2 12 5 4-90 24 High reverse breakdown undoped Lot face of single voltage and high for- No. 3081. crystal To. ward resistance.

17 200 ppm. C1 do 93. 5 4. 2 12 10 2-50 91 Aperture used over some doped under and I-V .c.) vacuum in characterisitics. laboratory.

ium rectifi claime in l i h rei We 61mm: 11 A selen er as d c a n1 1, w e n mate electrical connection with the second face of the single-crystal film of selenium. 2. A selenium rectifier as claimed in claim 1', and in which the single-crystal film is grown epitaxially on the said face of the first electrode.

3. A selenium rectifier as claimed in claim 2, and in t which:

(a) the said face of the first electrode is a single-crystal surface; and (b) the first electrode is so oriented that the grown single-crystal film of selenium has the same crystal the second electrode is doped to provide suitable differences between the forward and reverse electrical characteristics of the rectifier.

12. A selenium rectifier as claimed in claim 1, wherein the single-crystal selenium film is oriented in the (1010) plane.

References Cited UNITED STATES PATENTS 2,608,611 8/1952 Shive 3l7--241 2,675,509 4/1954 Barton 317-235 2,819,433 1/1958 Smith 3l7--241 2,914,837 12/1959 Herlet et al. 117-200 3,370,980 2/1968 Anderson 1l7--201 JAMES D. KALLAM, Primary Examiner U.S. Cl. X.R. 317234 

